1. Field of the Invention
The present invention is directed to a method and apparatus for facilitating the fabrication of new textile material preforms for fabrication into composite structures. In particular, the invention is directed to a method and apparatus for fabricating planar curved frame preforms that are later processed into a composite structure.
2. Description of the Background Art
Structural parts fabricated from composite materials are produced in a variety of methods: 1) hand layed-up prepreg; 2) filament winding; 3) tow placement and 4) dry fiber preforms (woven, braided and knitted).
No single fabrication method has proven optimal for all structures. One of the most complicated structures to fabricate from composite materials into a structurally efficient and cost effective part is a curved frame, such as the curved I-beam 200 in FIG. 1.
The difficulties in fabricating a curved frame, such as shown in FIG. 1, are: 1) maintaining the desired fiber orientation along the curved section; 2) uniform material coverage from inner to outer radial points along the curved section; 3) producing the required cross-sectional geometry and 4) cost.
The web 103 of a curved I-beam 200, for many applications, may have its fibers oriented in a + and - 45 degree direction relative to the circumferential axis (dotted line in FIG. 1 ). However, for other applications, the fiber angles in the web may need to vary as a function of circumferential position.
The above noted fabrication techniques have several drawbacks, as described below.
Hand lay up of tape or fabric prepreg is difficult and in many cases impossible to perform for a curved frame where control of fiber angle and material uniformity is critical. Considerable amount of cutting and darting of prepreg and the assembling of small pieces of prepreg are required to form the different elements of a curved frame. This process is labor intensive and the mechanical properties of the assembled elements are significantly below the theoretical values.
Filament winding is a continuous fiber placement technique whereby fibers are wrapped around a mandrel. This fabrication technique is not applicable to all geometrical shapes. For example, fiber bridging occurs along surfaces that have concave shapes. Furthermore, it is impossible to obtain a uniform coverage of material in a curved section using filament winding. Although filament winding is a highly automated technology, filament winding a complex shape multi-layer laminate can require considerable time due to the low material dispensing rate.
Tow placement is a technology similar to filament winding in that single/multiple tows or ribbons of material are dispensed onto a mandrel. Unlike filament winding, the tows can be discontinuous in length and tows can be placed along concave surfaces. Using discontinuous tows, it is possible to obtain near uniform coverage of material across a curved part and it is possible to maintain the desired fiber angle. However, the process can be slow and restricted to preimpregnated materials, and therefore expensive.
Weaving is a highly automated textile process for making dry fiber preforms. However, it is not currently possible to produce the unique fiber orientations and curved portion of the curved frame.
There are two types of braiding, 2-D and 3-D. In 2-D braiding a preform can be made whereby layers of dry fibers are braided around a mandrel. This fabrication method is limited to braiding around mandrels with convex surfaces. Concave sections of the mandrel's surface may produce fiber bridging. Another disadvantage of this method is the lack of uniform fiber coverage along the circumference of the section similar to that found in filament wound curved frames. Also, it is difficult to accommodate changes in fiber angle along the circumference while maintaining a uniform layer thickness.
It is conceptually possible, utilizing 3-D braiding, to fabricate a net-shape preform of a curved frame without braiding over a mandrel. Unfortunately, this technology has not significantly matured, and braiding time per pound of material is significant. Utilizing current 3-D braiding methods, the fiber architecture is highly crimped resulting in reduced mechanical properties.
Knitting is a high speed textile process that can be used to produce many complicated shapes. However, the knitting process creates an open structure resulting in a low fiber volume fraction and reduced mechanical properties. Because of these deficiencies, this technology has not been extensively exploited for application to composite structural preforms.
To facilitate the description of the important features of the present invention, the construction of a curved frame having an I cross-sectional shape will be described with respect to FIGS. 1 and 2.
If the I cross-sectional curved frame 100 was fabricated from prepreg or tow placement, it may be assembled as a set of nested C channels 101 with caps 102, as depicted in FIG. 2. The ideal fiber orientation of the web of the C channels 101 is generally + or - 45 degrees relative to the circumferential axis of the web 103 of the frame (See FIG. 1). The caps 102 contain a high percentage of 0 degree fibers with some 90 and possibly some + and - 45 degree fibers.